1. Field of the Invention
This invention relates to special effects produced on video images. The geometrical transformation involved in this process can be either a translation, an affine transformation, a rotation or an effect of perspective. If so required, a combination of these effects may also be contemplated.
2. Description of the Prior Art
A geometrical transformation of shapes can be applied to any video image scanned by a camera but is also applicable to an image which is generated electronically by means of devices known as image background generators. These devices generate a video signal corresponding to an image comprising, for example, colored rectangles on which credit titles are displayed. There also exists another type of device known as an electronic shape generator which, in conjunction with a device called a variable-gain switcher, makes it possible to combine two video signals in order to obtain an image in which certain portions are provided by one of the video signals whilst the other portions are provided by the other video signal. For example, a shape generator associated with a variable-gain switcher permits the formation of an image having a background which is supplied by a first video signal and comprises an inset with blurred edges supplied by a second video signal. It is desirable to have the capability of producing very different and variable shapes, according to the requirements of users.
It is a known practice to produce a vertical or horizontal compression or expansion of any video image by means of a digital process. For example, in order to produce a vertical image compression, the video signal is sampled at the standard sampling frequency of 13.5 MHz. Certain samples are then abandoned and the values of the remaining samples are written into a buffer memory having a capacity equal to one image. The write address values in the buffer memory are incremented at a mean rate which is lower than the sampling frequency. When the compression (packing) ratio is not equal to an integral value, the numerical values written into the buffer memory are determined by an interpolation. The buffer memory is then read by incrementing the read address values at a rate corresponding to the standard sampling frequency.
The performance of a rotation entails the need for more complex calculations and is thus difficult to achieve in real time. The method in accordance with the invention makes it possible to produce either a translation or a rotation or an affine transformation, or a combination of these elementary transformations, as well as to carry out a projection and thus to obtain an effect of perspective with a sufficient degree of simplicity of calculation to permit practical application in real time.
There are already a number of known methods for producing geometrical transformations on an image generated by an electronic shape generator associated with a variable-gain switcher. The method adopted depends on the type of generator employed. In order to produce signals for controlling the switcher, conventional shape generators comprise sawtooth-signal generators and parabolic signal generators. These signal generators can be either analog or digital generators. Analog analog generators have poor time stability because of temperature variations, tolerances of components, aging, and so on. Digital generators are not subject to the problem of stability of adjustments and are easier to adjust. However, the shapes obtained are not only of a simple type, but the geometrical transformations which are achievable in practice are limited in number. It is known, for example, to carry out a rotation of a square but not of a lozenge or of a star. It is also known to form blurred edges or colored borders, to perform a translation or a multiplication of patterns but without any possibility of effecting a rotation at the same time. The method in accordance with the invention makes it possible to overcome the disadvantages attached to electronic shape generators of known types.